The present invention relates to a communication system for hazardous areas and particularly to a system characterized by enhanced reliability.
Access by manufacturing or maintenance personnel to hazardous confined areas should be accompanied by particularly exacting safety precautions. In some states, a worker entering a toxic or flammable atmosphere is required to wear a safety harness with a lifeline attached by means of which a second worker will be immediately informed of an accident and will be able to effect rescue.
In some instances, where the complexity of a structure being manufactured does not permit the use of complex harnesses and the like, electronic communications equipment has been used in monitoring a worker's status. An advantageous system is set forth in Blevins et al, U.S. Pat. No. 4,331,953, entitled "Communication System for Use In Hazardous Confined Areas". According to this system, a worker entering a hazardous area carries a portable transmitter having a call-in button and an alarm button. As long as the worker periodically actuates the call-in button, and does not actuate the alarm signal, a central station will be notified of his safety. On the other hand if the worker either depresses the alarm button, or fails to call in within a predetermined time, an alarm is registered at the central station location such that rescue operations can be initiated. According to an additional feature of this system, the worker in the hazardous area is given a preliminary warning signal before his periodic call-in is due, to provide a reminder and prevent false alarms by unintended failure to report in. While the foregoing system is very effective, some difficulty has been experienced regarding false alarms, level of worker confidence, and the reluctance to spend the time required to use the system especially for short periods. In the prior system, the individual worker was not given first hand information on whether the equipment was working and he was actually being tracked by a central station, and whether information in the form of call-ins or alarm signals were being received. Sometimes a transmitter unit would be left at the entry of a hazardous area, i.e., one having close quarters, resulting in a false alarm at the end of a predetermined reporting period at which time the worker was either not available to report in, or could not hear the locally generated signal warning indicating the end of his call-in period was approaching. Furthermore, in some hazardous areas call-in would be prevented by structures shielding the transmission of radio waves causing a false alarm to be generated at the central station despite proper call-in procedure. The worker would be notified neither of reception of his call-in, his alarm, or of a false alarm condition if one occurred. Of course, periodic false alarms tend to lessen the significance of a legitimate alarm condition.
Furthermore, it was conventional in the prior system to provide a call-in period of approximately thirty minutes. Unfortunately in the worst case situation thirty minutes could elapse after a life threatening emergency before the central station was notified. However, requiring very frequent call-ins would be unduly burdensome and less likely to receive worker cooperation.
Emergency warning systems are known which are based upon detection of bodily movement. For example, a fireman may wear a motion detector system which emits an audible alarm if the fireman remains motionless for a predetermined time. The lack-of-motion condition may even be transmitted via radio to a second location. However, a motion sensor of this type provides no information if the unit is faulty, i.e., the lack of a warning therefrom does not necessarily indicate the absence of an emergency. Moreover, life threatening situations can occur in a manufacturing environment despite the presence of motion, e.g., on a moving structure or in the case of abnormal, involuntary movement on the part of the worker.